The invention relates to a grooved forming roll, which includes a rotably supported shell comprising circular openings arranged to be opened at least on the outer surface of the shell, and associated with each opening, a circular groove formed on the outer surface, arranged concentrically relative to the corresponding opening, and on which outer surface a portion of the formed circular grooves extends to each opening from each adjacent opening.
WO publication No. 9932713 discloses a forming roll and particularly the design of its outer surface. The object has been to increase the open surface of the shell for avoiding the previously used wire net. Previously, a plastic wire net was placed on top of the forming roll by shrinking, which wire net wore out fast during use. Another problem has been curling and soiling of the expensive wire net. The wire net has also set restrictions on the use of cleaning chemicals and high pressure cleaning. However, a drilled shell without a wire net would make clear marking in the web due to the effect of vacuum and/or the outer fabric during web forming. As a solution for the problem the publication proposes spiral grooves machined obliquely in relation to each other with which necks of material between the openings are divided into smaller parts. In this way, a flow connection is created between each two adjacent openings.
Machining of spiral grooves is a demanding procedure, in which special tools are required. In addition, despite of setting, the necks become indefinite in form and size. This makes the grooved outer surface of the shell sensitive to damaging and the sharp forms deplete the fabric. Furthermore, the proposed forming roll has operated in the desired way with some furnish grades only. The said publication also proposes the use of circular grooves in creating the grooving. Compared to spiral grooves, irrespective of easier machining, the remaining necks are large and the flow between the adjacent openings is poor. In this situation the marking problem remains as the flow speed of water varies in different positions of the forming roll.